Discipline: environment; Keywords: greenhouse gas, methane, biogenic cycle, fossil fuel gases, lifespan, livestock.
As carbon dioxide, methane is a potent greenhouse gas (GHG), but when it comes to ruminant livestock and climate change there are many other characteristics which need to be considered. The important ones are: (1) it stays in the atmosphere only for about 12 years versus 1000+ years for carbon dioxide; (2) it is derived from atmospheric carbon such as carbon dioxide; (3) it is part of the biogenic (photosynthetic) cycle, and (4) it eventually returns to the atmosphere as carbon dioxide, which means it is recycled carbon. In contrast to methane from ruminant livestock, methane from fossil fuels does not have all the same characteristics as biogenic methane. Apart from its short lifespan in the atmosphere, fossil methane shares more traits with carbon dioxide from fossil fuels in how it warms the planet, since it is not derived from atmospheric carbon but rather pulled from the earth, and therefore it is ‘new’ to the atmosphere. An article by the Clear Center at the University of California (7 July 2020) explains these concepts further. The reference is : https://clear.ucdavis.edu/expainers/why-methane-cattle-warms-climate-differently-co2-fossil-fuels, the title being: Why methane from cattle warms the climate differently than CO2 from fossil fuels.
As mentioned, methane stays in the atmosphere for about 12 years, where after 80-89 % is removed by oxidation with a process called hydroxyl oxidation. As a result, methane warms the earth only significantly for 12 years and because of the short lifespan, as it is emitted it is also destroyed in the atmosphere. Scientists referred to this characteristic as a flow gas. Thus, methane’s warming impact is not determined by how much is emitted, since it is destroyed relatively quickly, but by how much more is emitted compared to how much is destroyed. This implies that the amount being emitted can be equal to the amount being destroyed. For example, if a herd of cattle emits the same amount of methane over 12 years, they are contributing to warming for those 12 years. However, there after the amount which is emitted is the same as the amount being oxidised and thus the warming is neutral. Additional methane would of course alter the balance towards warming. For ruminant livestock farmers then, the intriguing aspect of biogenic methane is that if farmers are able to reduce the on farm methane emissions, and maintain the reduction, then a cooling effect can be created theoretically, since more methane is destroyed than what is emitted.
What is then the difference between biogenic methane and a fossil fuel GHG (example methane)? The biogenic methane from ruminant livestock begins as carbon dioxide which is already in the atmosphere, whereas gases which result from fossil fuel production begin deep in the earth, where they have been stored for millions of years away from the atmosphere, and they then simply add to the levels already in the atmosphere. The biogenic carbon dioxide in contrast, through photosynthesis, ends up in the plant which is eaten by ruminant livestock. During digestion a part is lost as methane which is emitted, enters the atmosphere and after 12 years is again converted to carbon oxide which again enters the biogenic cycle.
Comment: Until very recently the difference between biogenic methane and fossil fuel methane has not been understood and therefore not been recognized in calculations. Hopefully this will now be considered as it means the contribution of methane from ruminant livestock to global warming is much less and the contribution from fossil fuel much more in the total atmospheric load.